1. Field of the Invention
The present invention relates to the decrease of critical dimension of a semiconductor fabrication, and more particularly relates to a method that shrinks equivalent critical dimension of mask by In situ polymer deposition and etching on a photo-resist.
2. Description of the Prior Art
Critical dimension (CD) is a key factor of semiconductor device fabrication, it means the narrowest width that the photolithography process can achieve. Obviously, critical dimension is significantly decreased as scales of semiconductor products are continuously decreased.
In well-known techniques, methods for shrinking critical dimension can be divided into three principal categories: lithography method, photo-resist re-flow method and polymer deposition method. Where lithography method shrinks CD of semiconductor products by directly shrinking CD of mask and other methods shrinks CD of semiconductor products by shrinking CD of photo-resist.
Lithography method is the conventional method as FIG. 1A shows that pattern of mask 10 is totally transformed to photo-resist 11 that locates on substrate 12 by light 101 from a first light source, where the CD (W1) of mask 10 is equal to CD (W2) of photo-resist 11. The basic spirit of the method is that diffraction effect can be suppressed by reducing wavelength of light and resolution can be improved by reduction of wavelength of light. For example, 0.3 xcexcm CD corresponds to 365 nm wavelength, 0.2 xcexcm CD corresponds to 248 nm wavelength and 0.18 xcexcm CD corresponds to 193 nm wavelength. Therefore, as FIG. 1B shows, when the first light source is replaced by a second light source, pattern of mask 105 is totally transformed to photo-resist 115 that locates on substrate 126 by a light 102 from the second light source, where the CD (W3) of mask 105 is equal to CD (W4) of photo-resist 115. No matter how, owing to the fact that wavelength of light 102 is shorter than wavelength of light 11, the CD (W3) of mask 105 is smaller than the CD (W1) of mask 10.
In addition, two disadvantages of shrinking wavelength are inevitable: First, owing to the fact that depth of focus is proportion to wavelength, when thickness of photo-resist is non-negligible then short wavelength will induce non-uniform structure of structure of photo-resist and will decrease throughput of lithography process. Second, development of new light source with short wavelength is difficult, expensive and time-consuming. Beside, total lithography system must be re-designed for new short wavelength source.
In comparison, two rectified lithography methods are provided. One is phase shift mask method and the other is the off-axis illumination method. Both methods are shrinking CD by modifying property of mask such as structure and location, and it is not required to lower the wavelength of light.
In short, the phase shift mask method improves resolution of a mask by forming a shift layer on the mask to enhance constructive interference of light and reduce destructive interference of light. In comparison, off-axis illumination method improves resolution of mask by changing incidental angle between light and mask to prevent zero order diffract light vertically incident to a photo-resist and to change angle between different diffract lights. Obviously, both methods have a primary difficulty that required photography system is complicated and expensive.
Photo-resist re-flow method is a two-steps method to form narrow CD photo-resist by wide CD mask. First, as FIG. 1C shows, transforms pattern of mask 13 to a photo-resist 14 by light 135, where photo-resist 14 locates on substrate 15 and light 135 is emitted from a specific light source. Second, heat treats photo-resist 14 such that photo-resist 14 is melted and re-distributed, as shown in FIG. 1D. Obviously, by comparing FIG. 1C and FIG. 1D, during the heat treat process, each structure 17 of photo-resist 14 is filled by re-distributed surrounding photo-resist 14 and then CD of photo-resist 14 is smaller than CD of mask 13.
Polymer deposition method is another two-steps method to form narrow CD photo-resist by wide CD mask. First, light 175 from a specific light source is used to transform pattern of mask 17 to photo-resist 18 that covers substrate 19, as FIG. 1E shows. Second, as shown in FIG. 1F, polymer layer 193 is formed on photo-resist 18 and then each structure 196 of photo-resist 18 is filled by polymer layer 193. Therefore CD of photo-resist 18 is decreased by polymer layer 193. In addition, the useful method to form polymer layer 193 is the chemical mechanical deposition (CVD) method.
No matter how, both two-steps methods correspond to some disadvantages respectively. These disadvantages of photo-resist re-flow method comprise heat treat process and outline of structure 17 with re-distributed photo-resist 14 are difficult to control, effect of the method is not controllable and total fabricate process is increased. In comparison, disadvantages of polymer deposition method comprise effect of following etching process is decreased by polymer layer 193 that formed on bottom of structure 196, distribution of polymer layer 18 is difficult to be conformal and throughput of semiconductor fabrication is decreased by the forming process of polymer layer 193. Obviously, the advantage of these two-steps method that photo-resist with narrow CD is formed by mask with wide CD without reducing wavelength of light is cancelled by these disadvantages.
Therefore, according to previous paragraphs, it is indisputable that shrinking critical dimension is an important object in the development of semiconductor device fabrication, and it is particularly desired to develop a method that not only shrinks critical dimension but also prevents these previous disadvantages of well-known methods.
The primary object of the invention is to propose a method that efficiently shrink critical dimension of semiconductor device fabrication.
Another object of the invention is to form narrow CD photo-resist with wide CD mask. In other words, though CD of mask is not shrunk, but equivalent critical dimension is shrunk by shrinking critical dimension of photo-resist.
A further object of the invention is to avoid difficulties about development of narrow wavelength light source and improvement of photography system.
A still further object of the invention is to efficiently shrink CD of photo-resist when pattern of mask is totally and precisely transformed to photo-resist.
A yet further object of the invention is to propose a method not only shrinks CD of semiconductor device fabrication but also maintains throughput of semiconductor device fabrication in high level.
In order to achieve previous objects of the invention. A method comprises following essential points are proposed:
First step, forms a photo-resist on a substrate by a mask and a photolithography process, where pattern of the mask is totally and precisely transformed to the photo-resist and then CD of mask is equal to CD of photo-resist.
Second step, a plasma reactor with at least two independent power sources is used to form and etch a polymer layer on the photo-resist, where ion density and ion energy of plasma are adjusted respectively by different power sources.
Third step, adjusts voltages of all power sources such that etching rate and depositing rate are equivalent on surface of the photo-resist and etching rate is obviously larger than depositing rate in bottom of any structure of the photo-resist. Therefore, each structure is filled by a conformal polymer layer and then width of each structure is efficiently decreased. Thus, the critical dimension of the photo-resist is significant smaller than critical dimension of the mask. In other words, equivalent critical dimension of mask is shrunk by the invention. Beside, polymer layer does not be formed on bottom of any structure, and then following process such as etching is not obstructed by the polymer layer.
Indisputably, the idea of the invention is shrinking equivalent critical dimension of mask by In situ polymer deposition and etching on a photo-resist.
Obviously, the proposed method is an improved conventional polymer deposition method, and primary improvements comprise (1) both deposition and etching are used to form the polymer layer, (2) polymer layer does not be formed on bottom of any structure of the photo-resist.
Of course, when critical dimension of photo-resist is shrunk, any following process such as etching and deposition can be used to form semiconductor device with small critical dimension.